NMR and PET imaging will be used to follow post-radiation changes in the central nervous system of patients undergoing treatment of AVM's. Deep-seated AVM's, that are inaccessible to surgical therapy, are a cause of significant morbidity and mortality. In most patients, partial or complete obliteration of the lesion can be achieved by focal irradiation and stereotactically-directed He-ion beams have been used successfully for this purpose. However, the benefits of radiation are accompanied by the risk of radiation injury. Most studies of radiation injury have involved large-volume, photon irradiation and little is known about the risks of injury following focal high- LET irradiation. A group of over 200 AVM patients, being treated at the Lawrence Berkeley Laboratory, presents an opportunity to assess the CNS effects of focal He-ion irradiation, free of the effects of other pathology. NMR imaging and high-resolution (2.4 mm in-plane) PET imaging will be used to evaluate the AVM's, as well as surrounding tissue and tissue along the beam paths. Approximately 60 patients will be followed for 2-5 years and radiation response will be related to irradiatied tissue volume, as well as radiation dose. Other patients will be added to the study, when they develop clinical evidence of radiation injury. NMR has been shown to be sensitive for detection of some radiation effects, but is non-specific and cannot show the underlying physiologic changes. PET imaging with RB-82 (blood-brain-barrier), F-18-FDG (glucose metabolism) and I-122-HIPDM (regional blood flow) is specific for particular physiologic processes and the distribution of these tracers may explain some of the radiation effects noted.